Expert gives tips for maximizing growth of onions, tomatoes

If you cried last year because your garden’s onions didn’t quite match up to your expectations, take note: now is the time to do something about that.

“This is the time of year that onions grow and develop rapidly,” said Kansas State University horticulture expert Ward Upham. “Regular watering and a light fertilization are helpful to maximize growth.”

Upham suggests fertilizing with ammonium sulfate (21-0-0) at the rate of 1/2 cup per 10 feet of row if the garden’s soil tends to be alkaline. Otherwise, he said, “you can use a lawn fertilizer, but only use 1/3 cup per 10 feet of row.”

Lawn fertilizers with the label indicating 29-5-5 or 27-3-3 on the label are best. Upham said do not use lawn fertilizer that contain weed preventer or weed killer.

“Sprinkle the fertilizer two to three inches alongside the row and water in,” he said. “Do not fertilize after onions start to bulb.”

As onions develop, as much as 2/3 of the bulb remains out of the soil.

“This is normal and there is no need to cover the bulb with soil,” Upham said.


Most Kansas soils are warm enough now that tomatoes can benefit from mulching, so long as the soil is not saturated with water, according to Upham.

“Tomatoes prefer even levels of soil moisture, and mulches provide that by preventing excessive evaporation,” Upham said.

Other benefits of mulching include weed suppression, moderating soil temperatures and preventing the formation of a hard crust on the soil. Crusted soils restrict air movement into and out of the soil, slowing the water infiltration rate.

Upham said hay and straw mulches are popular for tomatoes, but may contain weed or volunteer grain seeds. Grass clippings can also be used, but should be applied in a thin layer, two to three inches.

“Clippings should also be dry, because wet clippings can mold and become so hard that the water can’t be passed through,” Upham said. “Also, do not use clippings from lawns that have been treated with weed killer until some time has passed. With most types of weed killers, clippings from the fourth mowing after treatment may be used.”

If the lawn was treated with a product containing quinclorac (such as Drive herbicide), the clippings should not be used as mulch.

Upham and his colleagues in K-State’s Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes.

Matt Young37 Posts

Matt Young is the Brown County Extension District director, as well as an agent in the area of agriculture and natural resources.


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